Methodology and integrity monitoring of foundation concrete piles using Bragg grating optical fibre sensors
Introduction
Condition assessment or health monitoring of structures is an integral part of civil engineering, where there is an ongoing need to estimate the response of structures under particular loading and environmental conditions [1], [2]. Health monitoring is also used as a diagnostic tool to detect or infer the presence of defects and to schedule maintenance operations. Assessment of civil structures can be performed by established techniques such as ultrasonic scanning [3], transient pulse and infrared thermography [4], [5] and ground radar [6], [7]. Although these conventional and established techniques are adequate and reliable, they are not suitable for real-time and in situ assessment of structures. These limitations can be overcome by using electrical resistance strain gauges [2], [8], acoustic emission [9], [10] or optical fibre sensors [11], [12], [13].
The advantages associated with optical fibre sensors over conventional monitoring techniques include their immunity to electromagnetic interference, small size and lightweight construction, and access to different measurands such as strain, temperature, vibration and specified chemicals. The optical fibre sensors can also be multiplexed, which means that more than one sensor can be integrated along a single optical fibre. This multiplexing capability can enable a distributed mapping of the structure to be monitored [14]. In order to perform long-term monitoring of structures and to ensure meaningful and reliable measurements, strategies for sensor protection and bonding have to be developed when the optical sensors are to be used in physically and chemically harsh environments.
This article reports on the monitoring of foundation piles by means of Bragg grating optical sensors. It describes the design strategy, material selection and the implementation of a sensor protection system for this structure to enable real-time and in situ acquisition of strain and temperature data. In this study, strain and temperature were monitored on two foundation piles during the whole construction phase of a 13-storey building at Bankside 123, London SE1, UK. This current paper also presents a detailed analysis of the monitoring of the piles during the pouring of the concrete into the boreholes and during the construction of the building floors.
Section snippets
Bragg grating sensors
A fibre Bragg grating (FBG) is a longitudinal periodic variation of the refractive index of the core of the optical fibre (Fig. 1). This refractive index modulation makes the Bragg grating a wavelength selective mirror. In that case, the grating reflects a narrow band of the incoming broadband spectrum that is centred at the Bragg wavelength, , determined by the following equation: where is the effective core refractive index and is the period of the grating.
The Bragg wavelength
Background to the project and site details
A number of the monitoring elements of this project were part-funded under the remit of a European Union project (EC 5th Framework project no EVK4-2001-00289). The overall aim of the project was to further the industry knowledge on the re-use of foundations for urban sites. Therefore, the attraction of an in situ sensor system capable of imparting real-time information on the relative magnitude of the strain and temperature: it will enable a realistic assessment of the structural integrity of
Background and requirements
The strategy was to attach the fibre Bragg grating strain and temperature sensors on T12 rebars in City University’s laboratory and then to secure these rebars on to the reinforcing cages on-site. The T12 rebars were welded to the different cage sections on-site. The rebar for the top section was 9 m long and those for the middle and bottom sections were 12 m long. The relative positions of the sensors in the reinforcing cages are presented in Fig. 3 along with the structure of the ground at
Bonding methodology of the FBG sensors and protection systems
The T12 rebar sections in question had a 0.5 mm ridge on opposite sides. The intention was to locate the optical fibre and the Bragg grating sensors along this ridge, as it offered a degree of additional protection. As far as the optical fibres inserted into the PTFE tubing were concerned, they were secured on to the rebar using the cyanoacrylate C2 adhesive (Permabond). Epoxy Araldite 2014 adhesive (provided by Vantico) was applied in the form of a 2 mm diameter bead on top of the pre-secured
Sensor interrogation unit
The different Bragg grating sensors bonded on the reinforcing cages were interrogated by means of an optical spectrum analyser (Anritsu), whose resolution was ±5 pm. The optical fibre pigtails were sequentially and manually connected to the analyser and the Bragg wavelengths were manually recorded. The 240 V power needed for the spectrum analyser was supplied by a petrol generator (Honda). The generator and spectrum analyser were transported to the site each time a sensor interrogation needed
Construction of the foundation piles
The extremities of the T12 rebars, on which the Bragg grating sensors were bonded, were welded onto the sections of the piling cage, and the lead-in and lead-out optical fibres were then coiled on a drum at the end of each section. When the different sections were completed, they were lifted horizontally by crane from the assembly site to the pile bore. The cage sections were then brought to vertical above the pile bore. In order to unroll the optical fibre leads and to attach them to one of
Results and discussion
Of the two foundation piles (referred to as pile 11 and pile 25 in the building plan) that were prepared for monitoring, only pile 11 was available for periodic monitoring. Its Bragg grating sensors were interrogated during the pouring and curing of the concrete and also during the construction of the different floors of the building.
Conclusions
The reinforcing cages of two concrete foundation piles were instrumented by means of Bragg grating strain and temperature sensors in order to evaluate the structural health of these piles for their eventual re-use in the construction of a future building. Sixteen optical sensors were bonded on the reinforcing cage of each foundation pile. In order to protect the sensors from the aggressive surrounding concrete and in order to obtain reliable measurements, a ‘clip-on’ protection system
Acknowledgements
This work was sponsored by the EPSRC under the Faraday-INTERsECT partnership. We would like to acknowledge Cementation Foundations Skanska for allowing access to the Bankside site. We would also thank Mr. M. Teagle for his valuable help during the fabrication of the sensor protection system.
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